The present invention relates to a channel assignment system for a loop network, particularly a loop network having a network control node, which allows a plurality of nodes to hold a duplex communication therebetween by time division circuit switching.
A channel assignment system applicable to duplex communication which is effected between a plurality of nodes in a loop network by time division circuit switching is described in, for example, "DATA AND COMPUTER COMMUNICATIONS" written by William Stallings (published by Macmillan), pp. 307-311 and 339-342. An example of such a system is shown in FIGS. 1-3.
Referring to FIG. 1, a prior art channel assignment system is shown which comprises a network control node, or center node, 100 and nodes 102-110. The network control node 100 and the nodes 102-110 are interconnected by a transmission path 111 to complete a loop network. The control node 100 includes a demand assign device 101 adapted to assign idle channels responsive to a communication request which may be sent thereto from any of the nodes 102-110. A frame with an exemplary channel arrangement is shown in FIG. 2. As shown, the frame includes an overhead 200 consisting of several bits and used for loop synchronization and frame synchronization purposes. The frame also includes channels 201-207. Generally, concerning circuit switching in a loop type network, a plurality of channels are set up in bits or bytes on a time division basis along a time base 208. In the illustrated example, the frame comprises the seven channels 201-207 and the overhead 200.
In FIG. 1, let the nodes 103, 104, 107 and 110 be called nodes A, B, C and D, respectively, and assume that all these nodes A-D ae capable of performing multi-station duplex communication. Then, as shown in FIG. 3, four idle channels are secured and assigned one to each of the nodes A-D. In this particular example, a channel 1 designated by the reference numeral 301 is assigned to the node A, a channel 2 designated by the reference numeral 302 to the node B, a channel 5 designated by the reference numeral 305 to the node C, and a channel 6 designated by the reference numeral 306 to the node D. Each of the nodes A-D reads pieces of information from the other three nodes through an exclusive read channel, while sending information to the other nodes on an exclusive write channel of its own.
The prior art channel assignment system as described above has the following problem. For N nodes on the loop network to hold a duplex communication therebetween, it is necessary to secure N idle channels. However, considering the topology particular to a loop, information is sequentially routed through the nodes according to a predetermined transmission direction of the loop, so that information transmitted from a certain node is practically valueless while it is returned from the last node in the loop, as viewed from the transmitted node, to the transmitted node. For example, in the case of digital picture transmission which needs a data rate of the order of 100 Mbps, meaningless information is undesirably propagated through the loop at the rate of 100 Mbps between the last node and the transmitted node. Such significantly limits the available transmission capacity of a loop network.